Stop being so interested in what the girl has to say.

Let's say a hot girl tells you she is a psychology major. Many guys would respond with "Wow that's awesome! Psychology is so interesting! I'm really into psychology!" They tend to over-enthusiastically agree with the girl, and any shared interests are causes for celebration.

As Aaron from EvolutionDaily pointed out, this behavior is how adoring fans typically behave for a celebrity they admire. In this case, the level of social value for each party is pretty distinct.

Toning down your interest will demonstrate to her that you are used to girls like her and that you are socially at the same level or higher. In this case a better response to the girl would be to match her level of enthusiasm and volume (or go a little bit below) and say something like "Cool, I'm into psychology as well. How did you get into it?"

Letting a girl know you are interested in her is hugely important to game. However, if you find that you are having trouble asserting yourself as high value from your conversations my recommendation would be to be a little less interested.